Yeah the .308 mag deal would work if you use the 100 grain bullets. Anything larger and you'd have to seat the bullet back farther into the case. Doable but not ideal.
THe mag length on those 7.5mm mags is more than adequate for the longest rounds. You could open up the throat and really do some work with it! For the cost, and the willingness to do a little tweaking on them, they are IMO the best mag for any of the straight-walled wildcats, etc. just so you use a PSL-length receiver. Now in a standard-length mag I would think a .30-06 mag conversion would be possible and that would give you the room for longer rounds like the 6.5x55. The receiver conversion steps are a known factor so if you can find some -06 mags of some sort (BAR mags perhaps?) then you would be able to use a standard-length stock or do the Saiga .308 conversion with no problems.
Edit - I found "brand new manufacture" BAR mags at OhioOrdnanceWorks.com for $25 each, so that is definitely doable!
I range tested the new Hornady A-Max ballistic tip bullets of different weights at the range and they all function fine. I see why my dad likes the 100 gr rounds, they are as fast as the 140gr at 2900 FPS and should be big enough for anything I'd ever encounter in N. America.